On a fresh installation LMDE 20124 Mate/Cinnamon on a Thinkpad T410s the internal speakers go mute.This happens with evolution notification sounds, flash videos in Firefox, during Skype calls or videos played in VLC or MPlayer.After reboot the speakers work for a while, then they go mute again.

Initially the same happened with the headset.After installing Pavucontrol, Pavumeter, etc. the sound on the plugged in headset works.

Nothing is muted in Alsamixer or Gnome Mixer. Gnome Mixer only recognizes the HDMI soundcard.And funny enough, Pavumeter shows sound being played,but the speakers don't make a sound.

Does anyone have an idea, what I am missing?Thanks in advance!

PS: No, I do not want to switch to XFCE/KDE/etc and I do not want to remove pulseaudio completely.

Update: Removing mono is one solution.According to quite a few forum posts elsewhere, Banshee does not work well with gnome3 andleads to hangups and freezes and mute speakers.

Update3:I was able to reproduce the problem byinstalling PulseAudio Preferences and adding a virtual device for multiple output.When two sounds are played (like music andan email notification, or a sound event whiledoing a Skype call) the speakers go mute.